1. Field of the Invention
The invention relates generally to borehole logging devices and, more particularly, but not by way of limitation, it relates to a logging sonde that utilizes high frequency acoustic energy to develop an indication of rock permeability for strata adjacent a borehole.
2. Description of the Prior Art
Previous attempts have been made at deriving permeability information from an acoustic energy borehole tool, and such prior systems generally relied upon detection of back-scattered acoustic energy to derive a meaningful indication. One patent of interest is the published United Kingdom patent application No. 2034888A in the name of Plona which involves the injection of a broad-band ultrasonic pulse containing 0.2 to 7.5 MHz frequencies into a subsurface formation and thereafter detecting the back-scattered acoustic energy. The back-scattered signal is then spectrally analyzed to give attenuation versus frequency data which is then equated to a grain size data or so called granularity property. This method assumes that there is an essential correlation of the attenuation of sandstone with its grain size, but certain experimental attenuation versus frequency data obtained by conventional transmission methods indicate that this may not be true.
Another prior art approach to the use of acoustic attenuation to obtain permeabilities involves Biot's theory of wave propagation in fluid-filled, porous solids. The extra increment of acoustic attenuation due to relative motion between the pore fluid and solid matrix is proposed as a measure of a formation's permeability. This method has been developed by Rosenbaum, 1974, Geophysics, Volume 39, No. 1, pp. 14-32. In general, frequencies used in application of the Rosenbaum method are lower than those used in the present invention and the dominant mechanism of attenuation is also different. Still other approaches utilize low frequency acoustic energy to generate and analyze tube wave data for borehole correlation as to permeability, e.g. U.S. Pat. No. 4,432,077.
Finally, U.S. Pat. No. 4,168,483 in the name of Parthasarathy describes a system for deriving data indicative of microfracture density in earth substructure adjacent to a borehole. Here again, the system analyzes back-scattered energy, i.e. energy reflected by the microfractures of the earth structure. The invention is directed toward the determination of permability and this must be presumed to be the contribution of microfractures to the formations permeability since there is no mention of the rocks matrix permeability, measurement of which is the aim of the present invention.